The present invention relates generally to a process and an apparatus for the production of hollow bodies from thermoplastic material, the wall of which comprises at least first and second layers, by means of extrusion blow molding.
In a process for the production of a hollow body from thermoplastic material with a wall in the form of a laminate structure comprising at least first and second layers, preforms which have a suitable number of layers are first produced in a batch-wise manner, using an extrusion unit comprising at least first and second extruders and a common extrusion head and which is provided with storage chambers for storage of the materials which are plasticised in the extruders, together with means for emptying of the storage chambers to form preforms, and flows of material, the number of which corresponds to the number of layers constituting the wall of the hollow body are brought together in the extrusion head in such a way that successively adjoining flows of material are joined together in a laminate-forming area to provide a laminate for constituting the wall of the preform, which is advanced from the above-mentioned laminate-forming area through a communicating duct within the extrusion head, to an outlet opening which is disposed at a spacing from the laminate-forming area. A given portion by volume of the laminate is then ejected through the outlet opening to form the preform which has two end portions and a central portion of which at least a part is expanded within the mold cavity of a divided extrusion blowing mold, under the effect of an increased internal pressure within the preform, while the end portions of the preform are squeezed off the preform, as constituting excess material, by means of a squeezing-off operation in which parts of the extrusion blowing mold are involved, with the excess material remaining outside the mold cavity of the mold.
German laid-open application (DE-OS) No 26 04 247 discloses a procedure in which the individual flows of material are firstly put into a configuration which is annular in cross-section, within the extrusion head, and the flows of material are then formed into the laminate which constitutes the preform, before the laminate leaves the extrusion head through the outlet opening. The laminate structure is formed during the operation of explusion of the stored materials to form the perform, the materials being expelled in that way by means of pistions. It is advantageous for the flows of material of all the layers to be brought together in a region which is very short as measured in the axial direction of the injection head. It is also possible for all the flows of material to be brought together to form the laminate structure, in a plane which is substantially normal to the longitudinal axis of the extrusion head.
The fact that the preform is formed in a batch-wise manner means that a distinction can be made between two operating phases within the extrusion head. After conclusion of the production of a preform by the ejection of a suitable amount of material through the outlet opening of the extrusion head, the above-mentioned storage chambers are emptied. In the subsequent phase of operation of the apparatus, each of the storage chambers is filled with material which is conveyed into the respectively appropriate storage chamber by a respectively associated plasticising unit, for example screw presses. In that sitution the pistons which are used for emptying of the storage chambers are moved back into their respective starting positions. During that phase of operation of the equipment, no preform is ejected from the extrusion head so that the material which is to be found in the duct system between the storage chambers and the outlet opening is not moved. After the operation of filling the storage chambers with the respective materials has been concluded, there then follows the second phase of operation in which the materials are ejected from the storage chambers under the effect of the pistons and are displaced towards the outlet opening. At the same time, the laminate structure is formed in the region in which the ducts carrying the flows of individual materials meet. From the laminate-formation area, the material then flows in the form of a laminate structure through a communicating duct towards the outlet opening. During that second phase of operation, the ejection procedure which provides for forming the preform is quite predominantly effected by the material being displaced out of the respectively storage chambers by the associated pistons. In addition that procedure involves the use of a smaller amount of material which, with the plasticising units operating continuously, is moved thereby towards the outlet opening during the ejection process.
The discontinuous production of performs results in different operating conditions, corresponding to the different phases of operation, in particular in regard to the pressure condtions obtaining in the extrusion head. Those different and accordingly varying pressure conditions mean that, at the beginning and/or at the end of each ejection operation, the pressure conditions obtaining in the laminate-formation region in the individual flows of material which go to make up the laminate structure differ, and that accordingly results in irregularities in the formation of the laminate structure. That is to be attributed for example to the fact that the volumes of the individual layers of the structure and therewith also the volumes of the individual storage chambers are normally different. As thermoplastic materials, in the plastic condition, do not behave like a liquid but within certain limits have a certain degree of resilient compressibility which, when the pressure is relieved, results in a return movement, that is to say an increase in the volume of the plastic material, it will be noted that, at the beginning and the end of each ejection stage, the laminate experiences undesirable deformation in the area of the apparatus in which the laminate structure is formed, due to differences in compression and compression-release effects occurring in the individual flows of material which go to make up the laminate. Such deformation of the laminate structure will result in irregularities in the laminate, in particular in regard to the spatial and quantitative distribution of the individual layers of the laminate structure. Those irregularities will occur in the first phase of the ejection operation as a condition of equilibrium will be restored, shortly after the beginning of the ejection operation. A similar aspect may also apply in regard to the terminal phase of the ejection operation. After the conclusion of the movement of the pistons which produce the ejection effect, the material experiences a relief of pressure and a decompression effect which will vary in relation to the individual layers of material and in the flows of material which make up the layers, by virtue of the differences in volume thereof, so that in this respect also irregularities in the distribution of the individual layers in the laminate structure may occur, in particular in the region of the part of the arrangement in which the laminate structure is formed, by virtue of the different pressure conditions obtaining therein. Those irregularities do not occur when preforms are extruded continuously as such a procedure does not give rise to variations in the pressure conditions, which are caused by batch-wise extrusion, with the attendant absence of laminate structure irregularities.
The above-mentioned irregularities which inevitably occur in batch-wise extrusion procedures can result in serious reductions in the level of quality of the articles to be produced from the preforms, by virtue of the fact that the walls thereof are of an irregular nature and configuration. In that connection the way in which the flows of material are guided in the extrusion head prior to the operation of forming the laminate structure is immaterial as the variations in the pressure conditions, which give rise to such irregularities, occur at any event due to the change between filling and emptying, which is typical in respect of batch-wise extrusion.